(a) Field of the Invention
The present invention relates to a three-dimensional 3-D sound system and a method thereof, and more particularly to a system and a method utilizing a head related transfer function (HRTF) for processing a two-channel signal to provide a 3-D sound effect.
(b) Description of the Related Art
A goal of a 3-D sound system is not only to reproduce the localization of the original sound sources but also to control the listener""s spatial auditory perception. To accomplish this, it is generally more effective to use 3-D sound technology in the recording (or encoding) process than in the reproducing (or decoding) process.
There has been significant study and research into implementing 3-D sound technology in recording, but the technology has not yet been applied to real recording systems. Two-channel stereo sound technology is still widely used in most recording systems to reproduce the sound source in audio, video, TV, etc.
On the other hand, some systems, including commercial theater and home theater sound systems, employ multi-channel reproducing methods (for example, Dolby, pro-logic, AC-3) to produce a 3-D sound effect. Generally, the multi-channel reproducing method mixes two-channel stereo signals with surround signals for a 3-D sound effect.
However, there is a drawback in such systems in that the use of the multi-channel reproducing method is limited to few recording systems, and there is a high cost associated with its implementation. As a result, many commercial sound systems are developed to implement a 3-D sound effect from two-channel stereo sources with two ordinary speakers. A prevailing method used in these systems is a stereo enhancement method.
An example of such a stereo enhancement method is described in U.S. Pat. No. 4,748,669. According to the stereo enhancement method, a sum signal (L+R) and a difference signal (Lxe2x88x92R or Rxe2x88x92L) are obtained from a stereo signal comprised of a left-channel signal (L-signal) and a right-channel signal (R-signal). The difference signal is dynamically enhanced to make the sound more spacious and deeper. That is to say, the stereo enhancement method analyzes the difference signal for each frequency band; and, if the magnitude is determined to be relatively small, the magnitude of the difference signal is increased and the magnitude of the sum signal is decreased, thereby realizing a sound with more depth and space.
However, the stereo enhancement method has many disadvantages. According to the method, the direction of the original sound source is distorted because it processes mixed signals, that is, both the sum and difference signals. Furthermore, processing mixed signals creates a mono-signal component which R and L-channels have in common, thereby creating a sound at the center of a listener. Therefore, when the channels of the original sound signal are widely separated, the resulting sound image is rather narrower than the original sound.
Further, an original sound signal which has been processed in many steps reproduces as an unnatural sound, which makes it difficult for an audience to listen for long periods of time.
In view of the foregoing, it is an object of the present invention to reproduce 3-D sound from a 2-channel stereo signal using a head related transfer function (HRTF).
It is another object of the present invention to maintain the direction of the original sound by processing signals for each channel separately.
According to one aspect of the present invention, to accomplish the above and other objects, each signal of 2-channel stereo signal is input to high-pass filter to remove the direct-current (DC) component. Each signal with removed DC component is processed by a finite impulse response (FIR) filter to produce a 3-D sound effect. The FIR filter implements the magnitude characteristic of the HRTF for a location adjustment. The FIR filters each receive an output signal from a high-pass filter and utilize a modified head related transfer function to relocalize a first position of a sound source to a second position, wherein the first position is an original position of the sound source and the second position is a target position of the sound source. Gain controllers are used to control gain of the signals output from the FIR filters.
In another aspect, a low-frequency compensation filter is used to compensate a low-frequency region of the output signals from the high-pass filters. A first adder is used to add output signals from the low-frequency compensation filter to the output of one of the FIR filters. A second adder adds output signals from the low-frequency compensation filter to the other FIR filter. Gain controllers control gain of output signals from the first and second adders.
In one embodiment, the two signals of the two-channel signal source correspond to left and right sides of a stereo signal. In one embodiment, the low-frequency compensation filter individually filters outputs from the high-pass filters. In another embodiment, the low-frequency compensation filter filters added outputs from the high-pass filters.
The HRTF is a spatial-filtering of a sound signal before it reaches the ear drum. Due to the asymmetry in the shape of the pinnae, when a single sound source is duplicated at a different position, a listener can recognize the position of each sound source because each sound source has a different HRTF.
According to another aspect of the present invention, a HRTF is properly modified and the modified HRTF is applied to a sound source such that a listener can recognize the predetermined location of the sound source, irrespective of its real location.
According to yet another aspect of the present invention, each signal with its DC component removed by a high-pass filter is applied to a low frequency compensation filter as well as to a FIR filter. At this time, the low frequency compensation filter compensates a low-frequency component lost during microphone recording, to maintain the direction of the recorded voice.